Brushless DC motors are employed in a wide variety of applications, and one application, for example, for brushless DC motors is as a spindle motor for a hard disk drive (HDD) or optical disk drive (i.e., digital versatile disk or DVD player). For some of these applications (like DVD players), speed control of the motor can be very important, as the motors will frequently change speed. This means that there are transient periods of braking and acceleration.
For braking, in particular, the motor should slow quickly to generally ensure that proper functionality is preserved, and, in FIG. 1, an example of a system 100-1 employs a braking scheme that can be seen. In this example, the motor 110 is a three-phase brushless DC motor, where each phase PHA to PHC is respectively coupled to transistor pairs Q1/Q2, Q3/Q4, and Q5/Q6 (which as shown are NMOS transistors) of inverter 106. The controller 104-1 applies pulse width modulation (PWM) signals PWM1 to PWM6 to the inverter 106 to control the phases PHA to PHC of the motor 110 (i.e., drive the motor 110). During braking, though, the motor 110 generates a reverse current or negative current through pins U, V, and W of integrated circuit (IC) or motor driver 102-1 to the supply pin VDD. When this occurs, the controller 104-1 closes switch S of discharge circuit 108 so as to activate the current mirror Q7 and Q8 (which, as shown, are PMOS transistors) by coupling the drain of transistor Q8 to the supply pin GND. This allows the reverse current or negative current to be discharged through resistor R2. One problem with this arrangement, however, is that transistors Q7 and Q8 can occupy a large portion of the area of IC 102-1 in order to be sufficiently large enough to carry the reverse current, so as an alternative (shown in FIG. 2), the discharge circuit 108 can be removed and several different types of braking schemes be employed (as shown in FIGS. 3 and 4).
For one scheme (which is shown in FIG. 3), controller 104-2 can inactivate or “turn off” transistors Q1 to Q6, placing the inverter 106 in a high impedance or HIZ mode. Mechanical friction (i.e., from bearings) can be used to slow the rotational speed of the motor 110. Usually, to allow this to occur, the speed command issued to the controller 104-2 changes from code L1 (which corresponds to a target rotational speed ω1) to code L3 (which corresponds to a target rotational speed that is not shown) at time T1 so as to allow a negative or reverse current to be generated. At this point, the inverter 106 is placed in a HIZ (off) state or mode, but the losses due to friction are usually so low that the motor 110 does not reach the desired target speed ω2 (which is associated with code L2) within the desired deceleration period (i.e., between times T1 and T2). Instead, the motor 110 reaches a much higher speed ω3 at time T2.
For another scheme (which is shown in FIG. 4), a short braking period can be employed. During the period between times T3 and T4, the speed command issued to controller 104-2 is set to code L3 (which corresponds to a target rotational speed that is not shown). As a result, the controller 104-2 places inverter 106 in a braking mode or state. In this braking state, transistors Q1, Q3, and Q5 are inactivated or “turned off,” while transistors Q2, Q4, and Q6 are activated or “turned on.” This allows a reverse or negative current to flow back through the pin COMM so as to be dissipated by resistor R1. This use of this short braking period is effective in slowing motor 110 to the desired or target speed within the desired deceleration period (i.e., between times T3 and T4), but the speed is not stable. There is some “ringing” that does occur.
Therefore, there is a need for an improved method and/or apparatus of braking with a brushless DC motor.
Some examples of conventional systems are: U.S. Pat. No. 6,528,968; U.S. Pat. No. 7,309,967; U.S. Pat. No. 8,098,031 U.S. Patent Pre-Grant Publ. No. 2009/0218972.